Small sonicated liposomes containing fluorescent compounds and cytotoxic drugs have been incubated with myeloma tumor cells, phagocytic cells other lymphoid cells. Dinitrophenol or phosphoryl choline hapten covalently bound to liposome membrane phospholipids mediates in vitro targeting of drugs to myeloma tumor cells bearing surface immunoglobulin of corresponding specificity. Binding of large quantities of cytotoxic drugs to non-phagocytic tumor cells has proven to be of little therapeutic value, because the drugs do not enter the cells after binding. By contrast, phagocytic tumor cells show markedly enhanced sensitivity to drug effects when the drug-containing liposomes are opsonized by antibody to the liposome bound hapten. These studies are being extended to evaluate the possibility of inducing specific immunologic unresponsiveness to haptens on drug carrying liposomes as a consequence of toxicity to precursor cells.